1. Field of the Invention
The present invention relates to the production of carbon monoxide from a carbonaceous source by reduction with hydrogen, and, more especially, to the preparation of gaseous mixtures containing hydrogen and carbon monoxide. The invention also relates to apparatus for carrying out the subject process, and various other applications thereof.
2. Description of the Prior Art
It is known to this art that H.sub.2 /CO mixtures are of considerable interest from commercial and technical points of view. Efforts have been made to produce such synthetic fuels for about the last three decades. However, these gaseous mixtures may also be useful in chemical processes such as the synthesis of ammonia, the synthesis of methanol, Fischer-Tropsch type syntheses, etc., as referred to in 1963, in U.S. Pat. No. 3,479,149.
Twenty years later, the attraction of such a transformation process has not decreased, but is quite to the contrary, both in the field of synthetic gases (also known as syngases), and in the field of synthetic fuels (known as synfuels). In fact, such reduction processing enables use of hydrogen, oxygen and CO.sub.2, it being appreciated in particular that, in this fashion, it is possible to "store" energy in liquid form. This is very important for, at the present time, electrical energy cannot be conserved, or cannot be satisfactorily conserved. One method comprises electrolyzing water and recovering the hydrogen and the oxygen, and re-using these elements in a conversion operation which ultimately results in a fuel.
In spite of the considerable amount of work which has resulted in a substantial amount of literature and the filing and issuing of many patents, it has been recognized that the following reaction: EQU CO.sub.2 +H.sub.2 .fwdarw.CO+H.sub.2 O
is generally carried out in a catalytic medium, for example, aluminosilicate (U.S. Pat. No. 3,479,149), ferric oxide (French Patent No. 2,295,118), Fe, Ni, Co and alloys thereof (published Japanese application No. 77/88,597), potassium carbonate (CA, Vol. 92, page 136, 92-166174j [1980]), and rhodium complexes (CA, Vol. 94, page 441, 94-128092q [1981]).
It has also been reported that an inert lining was sufficient for that reaction to be properly carried out (Actualite Chimique, January 1982, page 29). A plasma or a luminescent discharge too has been utilized (see, for example, CA, Vol. 87, page 135, 87-119989r [1977]).
By means of either of the aforesaid methods, by judicious selection among temperatures, pressures and the nature of the catalysts, it is possible to achieve the formation of hydrocarbons; see, for example, the above French Patent No. 2,295,118. It is also possible to prepare methane from CO.sub.2 by luminescent discharge in the presence of Fe to produce carbon monoxide, and then by reduction of the CO in the presence of a catalyst comprising Co-ZrO.sub.2 MgO- kiesselghur, at 190.degree. C., to give methane (CA, Vol. 90, page 537, 90-103291h [1979]).
The above review which is not intended as exhaustive reflects that the topic reduction operation is not an easy one to master. Even when using a burner at a temperature in excess of 600.degree. C., a catalyst was used (CA, Vol. 90, page 140, 90-189269s [1979]).
It too is known that the rate of conversion of CO.sub.2 to CO is low in the usual catalysis temperature range, irrespective of the mode of use of the catalyst (fixed bed, fluidized bed, entrained bed, etc.), that defect being accentuated in the case of a fluidized bed by virtue of short-circuiting because of the bubbles.
Under the circumstances, that disadvantage is in addition to the conventional deficiencies of the catalysts (resistance in respect of time to temperature and corrosion, impurities, cost, and the like).
It has also been proposed that the subject reaction may be carried out at a temperature of more than 800.degree. C. in the presence of a chemically inert reactor lining (Actualite Chimique, January 1982, page 29).
However, such mode of operation suffers from a number of disadvantages:
(1) The chemically inert nature of the lining can be preserved only insofar as the reactants do not comprise any active substance which is liable to be deposited directly onto the liner and no active substance is formed over the course of any secondary reactions; and
(2) In addition, the lining is homogenous in respect of temperature only insofar as it is heated by an equally distributed exothermic reaction. It is known that such a lining has both radial and longitudinal temperature profiles with hot spots towards the center and cooled regions in the vicinity of the periphery if the heat required is generated in situ, such temperature profiles being reversed in the case where heat is supplied through the walls thereof. The flow section corresponding to the annular regions which are close to the wall being very large, a large portion of the gas flow therefore takes place under temperature conditions which are different from those prevailing in the main volume of the apparatus, which corresponds to different states of kinetics in respect of the equilibria involved and possible secondary reactions as described below.
Moreover, the temperature profile mentioned above is particulary detrimental at the moment of start-up and more generally in any variation in operating manner, which may result in deposits of soot.
In fact, it is known that, at a temperature of less than 800.degree. C., any CO which may be formed is degraded to form undesirable products (for example, carbon, by Boudouard reaction).
This implies that the reaction should be carried out at a temperature in excess of 800.degree. C. and in a homogeneous gaseous phase. However, it has also been claimed that it is necessary for the operating temperature to exceed 1300.degree. C., in order to establish an appreciable rate of evolution of equilibrium conditions. On the other hand, calculations evidence that, above a temperature on the order of 1200.degree. C., the energy required for increasing the rate of conversion of CO.sub.2 to CO becomes prohibitive, which detracts from the desirability of a process using a plasma or a luminescent discharge.
However, raising a gas to a temperature of more than 800.degree. C. (and a fortiori to more than 1300.degree. C.) by indirect heating gives rise to technological difficulties which are well known (hydrogen diffusion, and fatigue in and carburization of metal alloys, and the like) and presupposes providing a hot source at a temperature which is markedly higher than that of the gas (thermal efficiency and resistance of the materials involved).
It has been reasoned that the immediately above should direct attention in favor of reactors in which the products react and are transported solely in a gaseous phase, but among the problems which then arise, those encountered include contacting the reactants, and transferring mass and heat. The contacting of the reactants must be carried out by rapid mixing at very high temperatures in order to avoid local and transitory degradation. However, and in particular from a technological point of view, it is an attractive proposition to use a reactor of cooled wall type, namely, a reactor having sidewall temperature which is lower than the tolerances of conventional steels (800.degree. C.), whereas the reaction temperature is generally much higher. Nonetheless, that reactor must not suffer from the aforementioned disadvantage, i.e., localized cooling of the gases by the reactor wall. It must therefore have a very low surface area/volume ratio (no lining) and must permit a high level of production per unit of volume.
In consequence, raising the mixture of the reactants to such temperatures very quickly and homogenously, in respect of temperature and composition, implies in situ generation of heat.
For that purpose, consideration has been given to use of rocket motor technology, by maintaining a boundary layer of unreacted hydrogen along the walls of the reaction zone. However, this is a contradiction in terms with respect to the requirements of a medium which is of maxium homogeneity.
An ideal reactor would consist of a reactor which has both relatively cold sidewall members and a reaction medium temperature identical to the theoretical temperature.